Fungi are responsible for devastating epidemics which can lead to considerable losses of crops of various plant species. The principle of employing inhibitors of enzymes from pathogenic fungi, and of using these enzymes in tests for identifying novel molecules which are active against these fungi is known per se. However, simple characterization of a fungal enzyme is not sufficient to achieve this aim, the enzyme chosen as a target for potential fungicidal molecules also has to be essential to the life of the fungus, its inhibition by the fungicidal molecule resulting in death of the fungus, or essential to the pathogenesis of the fungus, its inhibition not being lethal for the fungus, but simply inhibiting its pathogenic potency. The identification of metabolic pathways and of enzymes essential to the pathogenesis and to the survival of the fungus is therefore necessary for the development of novel fungicidal products.
Ketol-acid reductoisomerase is an enzyme which has been well characterized in plants and microorganisms such as bacteria and yeast. This enzyme is the second enzyme of the biosynthetic pathway for branched-chain amino acids; it catalyzes conversion of the substrate 2S-2-acetolactate (AL) or 2S-2-aceto-2-hydroxybutyrate (AHB) to 2,3-dihydroxy-3-isovalerate (DHIV) or to 2,3-dihydroxy-3-methylvalerate (DHIM), respectively. This reaction requires the presence of magnesium ions (Mg2+) and occurs in two steps: isomerization of a methyl or ethyl group, followed by reduction by NADPH. A great deal of knowledge has been acquired regarding plant reductoisomerase as a target for herbicides (Wittenbach et al., Plant Physiol. 96, No. 1, Suppl., 94, 1991; Schulz et al., FEBS Lett., 238:375–378, 1988) and ketol-acid reductoisomerase inhibitors have been described as herbicides (EP106114; U.S. Pat. No. 4,594,098, EP196026, EP481407, WO 94/23063, CA2002021). However, these compounds have not shown effective herbicidal activity on plants.
A subject of the present invention is methods for treating crops against fungal diseases, comprising applying a ketol-acid reductoisomerase inhibitor. It has been found that inactivation of the ILV5 gene encoding ketol-acid reductoisomerase in Magnaporthe grisea results in inhibition of fungal growth. This inhibition of fungal growth is also observed in vivo in the presence of inhibitors specific for ketol-acid reductoisomerase. M. grisea is pathogenic for many species of crop plants such as rice.